1. Field of the Invention
The present invention relates generally to earth boring bits used to drill a borehole for the ultimate recovery of oil, gas, or minerals. More particularly, the invention relates to rolling cone and percussion rock bits, and to an improved cutting insert for such bits. Still more particularly, the invention relates to enhancements in insert geometry and in the interface between an insert substrate and a wear-resistant coating.
2. Description of the Related Art
An earth-boring drill bit is typically mounted on the lower end of a drill string and is rotated by revolving the drill string at the surface or by actuation of downhole motors or turbines, or by both methods. With weight applied to the drill string, the rotating drill bit engages the earthen formation and proceeds to form a borehole along a predetermined path toward a target zone. The borehole formed in the drilling process will have a diameter generally equal to the diameter or “gage” of the drill bit.
A typical earth-boring bit includes one or more rotatable cone cutters that perform their cutting function as they roll and slide upon the bottom of the borehole as the bit is rotated, the cone cutters thereby engaging and fracturing the formation material in their path. The rotatable cone cutters may be described as generally conical in shape and are therefore referred to as rolling cones or rolling cone cutters.
Rolling cone bits typically include a bit body with a plurality of journal segment legs. The rolling cones are mounted on bearing pins or shafts that extend downwardly and inwardly from the journal segment legs. The borehole is formed as the gouging and scraping or crushing and chipping action of the rotary cones removes chips of formation material which are carried upward and out of the borehole by drilling fluid which is pumped downwardly through the drill pipe and out of the bit.
The earth disintegrating action of the cone cutters is enhanced by providing the cone cutters with a plurality of cutter elements. Cutter elements are generally of two types: inserts formed of a very hard material, such as tungsten carbide, that are typically press fit into undersized apertures in the cone surface; or teeth that are milled, cast or otherwise integrally formed from the material of the rolling cone. Bits having tungsten carbide inserts are typically referred to as “insert bits” or “TCI bits,” while those having teeth formed from the cone material are commonly known as “steel tooth bits.” In each instance, the cutter elements on the rotating cone cutters breakup the formation to form new borehole by a combination of gouging and scraping or chipping and crushing.
In oil and gas drilling, the cost of drilling a borehole is proportional to the length of time it takes to drill to the desired depth and location. The time required to drill the well, in turn, is greatly affected by the number of times the drill bit must be changed before reaching the targeted location. This is the case because each time the bit is changed, the entire string of drill pipes, which may be miles long, must be retrieved from the borehole, section by section. Once the drill string has been retrieved and the new bit installed, the bit must be lowered to the bottom of the borehole on the drill string, which again must be constructed section by section. As is thus obvious, this process, known as a “trip” of the drill string, requires considerable time, effort and expense. Accordingly, it is always desirable to employ drill bits which will drill faster and longer and which are usable over a wider range of formation hardness.
The length of time that a drill bit may be employed before it must be changed depends upon its ability to “hold gage” (meaning its ability to maintain a full gage borehole diameter), its rate of penetration (“ROP”), as well as its durability or ability to maintain an acceptable ROP. The geometry and positioning of the cutter elements upon the cone cutters greatly impact bit durability and ROP and thus, are critical to the success of a particular bit design.
Conventional cutting inserts typically have a body consisting of a cylindrical grip portion that is retained in the rolling cone cutter, and a cutting portion that extends from the grip portion and engages the formation material. These inserts are typically inserted in circumferential rows on the rolling cone cutters. Most such bits include a row of inserts in the heel surface of the cone cutters. The heel surface is a generally frustoconical surface and is configured and positioned so as to align generally with and ream the sidewall of the borehole as the bit rotates.
In addition to the heel row inserts, conventional bits typically include a circumferential gage row of cutter elements mounted adjacent to the heel surface but oriented and sized so as to cut the corner of the borehole. In performing their corner cutting duty, gage row inserts perform a reaming function, as a portion of the insert scraps or reams the side of the borehole. Gage row inserts also perform bottom hole cutting, a duty in which the inserts gouge the formation material at the bottom of the borehole.
Conventional bits also include a number of additional rows of cutter elements that are located on the cones in circumferential rows disposed radially inward or in board from the gage row. These cutter elements are sized and configured for cutting the bottom of the borehole, and are typically described as inner row cutter elements.
A variety of different shapes of cutter elements have been devised. In most instances, each cutter element is designed to optimize the amount of formation material that is removed with each “hit” of the formation by the cutter element. At the same time, however, the shape and design of a particular cutter element is also dependent upon the location in the drill bit in which it is to be placed, and thus the cutting duty to be performed by that cutter element. For example, heel row cutter elements are generally made of a harder and more wear resistant material, and have a less aggressive cutting shape for reaming the borehole side wall, as compared to the inner row cutter elements where the cutting duty is more of a gouging, digging and crushing action. Common geometries for inner row cutter elements are chisel or conical shapes.
It is understood that cutter elements, depending upon their location in the rolling cone cutter, have different cutting trajectories as the cone cutter rotates in the borehole. Thus, conventional cutter elements have been oriented in the rolling cone cutters in a direction believed to cause optimal formation removal. However, it is now understood that cutter elements located in certain portions of the cone cutter have more than one cutting mode. More particularly, cutter elements in the inner rows of the cone cutters, especially those closest to the nose of the cone cutter (and the center line of the bit), include a twisting motion as they gouge into and then separate from the formation. Unfortunately, however, conventional cutter elements, such as a chisel shaped insert, having a single primary cutting edge, are usually oriented to optimize the cutting that takes place only in the cutter's circumferential cutting trajectory, as they do not have particular features to take advantage of cutting opportunities arising from the twisting motion of the cutter element.
Accordingly, to provide a drill bit with higher ROP, and thus to lower drilling costs incurred in the recovery of oil and other valuable resources, it would be desirable to provide cutter elements designed and oriented so as to enhance brittle fracture of the rock formation being drilled, and to present to the formation multiple cutting edges as the cutting surface of the cutter element rotates through its cutting trajectory so as to take advantage of multiple cutting modes.
At the same time, it is desirable to make the inserts wear-resistant so as to increase the useful life of the bit and decrease the numbers of times the bit must be replaced. In order to improve their operational life, these inserts are preferably formed from a substrate body that is coated with an ultrahard and wear-resistant material, such as a layer of polycrystalline diamond, thermally stable diamond or any other ultrahard material. The substrate, which supports the coated cutting layer, is normally formed of a hard material such as tungsten carbide (WC). The basic techniques for constructing polycrystalline diamond enhanced cutting elements are generally well known, and can be summarized as follows: a carbide substrate is formed having a desired surface configuration and then placed in a mold- with a- superhard material, such as diamond powder and/or its mixture with other materials which form transition layers, and subjected to high temperature and pressure, resulting in the formation of a diamond layer bonded to the substrate surface.
Despite the advantages and improvements provided by diamond coated inserts, such inserts sometimes fail in use. In particular, it has been found difficult to employ diamond coated inserts on the inner rows of rolling cone rock bits where they must endure substantial impact loads as the cutting inserts gouge and cut the borehole bottom. One typical failure mode is caused by internal stresses, for example thermal residual stresses resulting from the manufacturing process, which tend to cause delamination between the diamond layer and the substrate or the transition layer, either by cracks initiating along the interface and propagating outward, or by cracks initiating in the diamond layer surface and propagating catastrophically along the interface. One explanation for such failures is that the interface between the diamond and the substrate or a transition layer is subject to high residual stresses resulting from the manufacturing processes of the cutting element. Specifically, because manufacturing occurs at elevated temperatures, the differing coefficients of thermal expansion of the diamond and substrate material or transition layer result in thermally-induced stresses as the materials cool down from the manufacturing temperature. These residual stresses tend to be larger when the diamond/transition-layer/substrate interfaces have smaller radii of curvature. In part for this reason, where diamond coated inserts have been employed in certain formations, the inserts are typically formed with the carbide/diamond interface surface having a relatively large radii of curvature and uncomplicated geometries, such as generally hemispherical shaped tops or relatively blunt chisel shapes. At the same time, as the radius of curvature of the interface increases, the application of cutting forces due to contact of the formation on the cutter element produces larger stresses at the interface, which can enhance the detrimental effects of the residual stresses and result in delamination.
The primary approach used to address the delamination problem in convex cutter elements is the addition of transition layers between the ultrahard material layer and the substrate, applied over the entire substrate interface surface. These transition layers are made of materials with particular thermal and elastic properties and tend to reduce the residual stresses at the interface, thus improving the insert's resistance to delamination. U.S. Pat. No. 6,315,065, commonly owned by the assignee of the present patent application, describes certain inserts and transition layers and is hereby incorporated by reference in its entirety. Nevertheless, residual stresses cannot be entirely eliminated and still cause insert failure.
More specifically, the residual stresses, when augmented by the repetitive stresses attributable to the cyclical loading of the cutting element by contact with the formation, may cause spalling, fracture and delamination of the diamond layer from the transition layer or the substrate. In addition to the foregoing, state of the art cutting elements often lack sufficient diamond volume to cut highly abrasive formations, as the thickness of the diamond layer tends to be limited by the resulting high residual stresses and the difficulty of bonding a relatively thick diamond layer to a curved substrate surface even with the employment of the transition layers. Hence, it is desired to provide a cutting element that provides increased bit life, and that enhances the cutting insert's ability to resist spalling, delamination and failure modes caused or accelerated by residual stresses.